Hydraulic fluids in plastic injection molding processes

ABSTRACT

The present invention relates to the use of hydraulic fluids in plastic injection molding processes. Thereby it was surprisingly found that the use of hydraulic fluids with the right combination of physical parameters like the viscosity grade, the viscosity index, the density and the dispersancy allows for significant energy savings in plastic injection molding processes (PIM). The PIM process is an industrial process to manufacture plastic parts at well controlled temperatures, pressures and cycle times. The energy consumption of the process became more important over the last years, however, other parameters like process stability and accuracy of plastic part parameters as well as machine protection and long oil drain intervals have to be satisfying.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to the use of hydraulic fluids in plasticinjection molding processes. Thereby it was surprisingly found that theuse of hydraulic fluids with the right combination of physicalparameters like the viscosity grade, the viscosity index, the densityand the dispersancy allows for significant energy savings in plasticinjection molding processes (PIM). The PIM process is an industrialprocess to manufacture plastic parts at well controlled temperatures,pressures and cycle times. The energy consumption of the process becamemore important over the last years, however, other parameters likeprocess stability and accuracy of plastic part parameters as well asmachine protection and long oil drain intervals have to be satisfying.

BACKGROUND OF THE INVENTION

Injection molding is a manufacturing process for producing parts byinjecting material into a mold at well controlled temperatures,pressures and cycle times. Injection molding can be performed with ahost of materials, including metals, glasses, elastomers, confections,and most commonly thermoplastic and thermosetting polymers. Material forthe part is fed into a heated barrel, mixed, and forced into a moldcavity, where it cools and hardens to the configuration of the cavity.

The power required for this process of injection molding depends on thevarious movements in the molding machine, but also varies betweenmaterials used. The book Manufacturing Processes Reference Guide fromRobert Todd states that the power requirements depend on a material'sspecific gravity, melting point, thermal conductivity, part size, andmolding rate. Injection molding machine is actuated by hydraulic system,wherein the electrical energy is transformed into mechanical energythrough hydraulic energy. The energy reaches the actuators in the formof pressure and volume flow. While transmitting power through hydraulicforces, a loss of energy is observed due to flow losses and friction. Inaddition, the compression of hydraulic fluid develops frictional heat,which has to be controlled for example by cooling. Pump type and controlof that pump also contribute heavily to how efficient a molding machineis in processing the plastic.

In the state of the art some efforts were made to save energy bymodification of the injection molding machines. In EP 0 403 041 forexample special alternating-current servo motors for the pumps which areconnected to the hydraulic consumers are used. In U.S. Pat. No.4,020,633 a completely new concept for the whole hydraulic drive systemof the injection molding machine is disclosed. But none of theseconcepts touches the hydraulic fluid that is used here. Therefore itmust be possible to realize additional energy savings by optimizingthese fluids.

EP 2337832 discloses a method of reducing noise generation in ahydraulic system, comprising contacting a hydraulic fluid comprising apolyalkyl(meth)acrylate polymer with the hydraulic system. The hydraulicfluid has a Viscosity Index VI of at least 130. Thepolyalkyl(meth)acrylate has a molecular weight in the range of 10 000 to200 000 g/mol and is obtained by polymerizing a mixture of olefinicallyunsaturated monomers, said mixture comprising preferably 50 to 95 wt %C₉ to C₁₆ and 1 to 30 wt % of C₁ to C₈.

Target of the invention described in EP 2337832 was the reduction ofnoise which is achieved by increasing oil viscosities at highertemperatures. For this effect high viscosities and high densities arebeneficial and the high VI of the fluids is used to increase theviscosity at the operating temperature.

In the present invention a completely different approach is used toincrease the energy efficiency. A high VI is used to enable a reductionof the base fluid viscosity. This reduced viscosity in combination witha low density of the hydraulic base fluid increases the efficiency ofthe injection molding process. In comparison to EP 2337832 it is notexpected that hydraulic fluids according to the present inventiondecrease the noise level.

EP 2157159 discloses a hydraulic fluid containing, as a base oil, anester containing at least two ring structures. It is described that thehydraulic fluid has low energy loss due to compression and exhibitsexcellent responsiveness when being used in a hydraulic circuit.Consequently, the hydraulic fluid realizes energy-saving, high-speedoperation and high precision of control in the hydraulic circuit.

EP 1987118 discloses the use of a fluid with a viscosity improving indexof at least 130 for the use in hydraulic systems like engines orelectric motors. This fluid comprises a copolymer of C₁ to C₆(meth)acrylates, C₇ to C₄₀ (meth)acrylates and optionally further with(meth)acrylates copolymerizable monomers in a mixture of an API group IIor III mineral oil and a polyalphaolefine with a molecular weight below10,000 g/mol. It is neither shown here that such a fluid is also usablein an injection molding machine nor which specific composition of thefluid would be applicable in such a machine.

However, there still exists a need to investigate further on possiblealternative hydraulic fluid compositions to be used in a hydraulicsystem subject to high working pressure, like in plastic injectionmolding processes.

OBJECT

The improvement of energy efficiency is a common object in the technicalfield of injection molding. Usually such objects are achieved byconstruction improvements of the unit providing mechanical energy of thehydraulic system. However, there is still a need for furtherimprovements with regard to that object. Accordingly, the purpose of thepresent invention was to provide a method for saving energy, increaseproductivity, avoid heating, improve air release and avoid cavitationover a broad temperature operating window in a hydraulic system used inplastic injection molding processes.

Especially was the object of the present invention to improve theperformance of a hydraulic system in a plastic injection molding machinewith energy savings of at least 5% and of up to 10%, compared to theperformance of a machine when run with a standard fluid having a VIaround 100 as recommended by the producers of injection moldingmachines. It was also object to realize an energy saving for single,very energy consuming process steps of more than 10%.

Especially it was the object of the present invention to realize thisenergy saving by providing a new hydraulic fluid for the use in plasticinjection molding machines.

Further objects not explicitly discussed here may become apparent hereinbelow from the prior art, the description, the claims or exemplaryembodiments.

DESCRIPTION OF THE INVENTION

The above-indicated prior art documents relating to injection moldingprocesses try to reduce energy consumption, but without changing oilparameters. After an exhaustive investigation, the inventors haveunexpectedly found that the hydraulic fluid plays a crucial role forsaving energy in plastic injection molding processes, and in particularthat some hydraulic fluid compositions adjusted to the right physicalparameters, allow for energy savings of up to 5% or more in the overallplastic injection molding process (PIM), or more than 10%, mostly up to15% for certain step of the PIM process. Indeed, by adjusting theviscosity grade, the viscosity index, the density and dispersancy of thehydraulic fluid as defined in claim 1, the inventors have found that asignificant amount of energy can be advantageously saved, even byoperating at high pressure conditions as it is usual in PIM processes.

In detail, the objects discussed above have been solved by a novelmethod of reducing the energy consumption of a hydraulic system in anindustrial hydraulic application, preferably in a plastic injectionmolding process or in a process comprising a hydraulic press. In thismethod a hydraulic fluid is used in a plastic injection molding process.The hydraulic fluid composition thereby comprises (i) apolyalkyl(meth)acrylate viscosity index improver and (ii) a base oil.

The polyalkyl(meth)acrylate viscosity index improver (i) therebycomprises at least monomer units a) and b) and optionally monomer unitsc) and/or d). Preferably the component (i) has a weight averagemolecular weight (M_(w)) from 20,000 to 100,000 g/mol. More preferredthe molecular weight M_(w) is between 30,000 and 85,000 g/mol andespecially preferred between 40,000 and 70,000 g/mol. The polydispersityindex of the polyalkyl(meth)acrylate viscosity index improver is between1 and 4, preferred between 1.2 and 3.0 and most preferred between 1.5and 2.5.

The polyalkyl(meth)acrylate viscosity index improver (i) contains 5 to40 wt. %, preferred 7 to 30 wt. %, more especially preferred 10 to 25wt. % of repeating units that have been obtained by the copolymerizationof monomers a) and 50 to 95 wt. %, preferred 60 to 93 wt. %, moreespecially preferred 70 to 90 wt. % of repeating units that have beenobtained by the copolymerization of monomers b). In a special embodimentof the invention the amount of the compound of formula (II) is between75 and 90 wt. %, especially preferred between 70 and 80 wt. %.

Monomers a) thereby are one or more ethylenically unsaturated estercompounds of formula (I)

wherein R is equal to H or CH₃, R¹ represents a linear or branched alkylgroup with 1 to 6 carbon atoms and R² and R³ independently represent Hor a group of the formula —COOR′, wherein R′ is H or an alkyl group with1 to 5 carbon atoms.

Examples of component a) are, among others, (meth)acrylates, fumaratesand maleates, which derived from saturated alcohols such as methyl(meth)acrylate, ethyl (meth)acrylate, n-propyl (meth)acrylate, isopropyl(meth)acrylate, n-butyl (meth)acrylate, tert-butyl (meth)acrylate and/orpentyl (meth)acrylate; cycloalkyl (meth)acrylates, like cyclopentyl(meth)acrylate. Methacrylates are even preferred over acrylates.

Monomers b) are one or more ethylenically unsaturated ester compounds offormula (II)

wherein R is equal to H or CH₃, R⁴ represents a linear or branched alkylgroup with 7 to 15 carbon atoms and R⁵ and R⁶ independently represent Hor a group of the formula —COOR″, wherein R″ is H or an alkyl group with6 to 15 carbon atoms.

Among these are (meth)acrylates, fumarates and maleates that derive fromsaturated alco-hols, such as n-hexyl (meth)acrylate, 2-ethylhexyl(meth)acrylate, heptyl (meth)acrylate, 2-tert-butylheptyl(meth)acrylate, octyl (meth)acrylate, 3-isopropylheptyl (meth)acrylate,nonyl (meth)acrylate, decyl (meth)acrylate, undecyl (meth)acrylate,5-methylundecyl (meth)acrylate, dodecyl (meth)acrylate, 2-methyldodecyl(meth)acrylate, tridecyl (meth)acrylate, 5-methyltridecyl(meth)acrylate, tetradecyl (meth)acrylate and/or pentadecyl(meth)acrylate.

The polyalkyl(meth)acrylate viscosity index improver (i) may alsocontain further components that are in form of a monomer copolymerizablewith at least one of the components a) and b). These further monomersare especially the components c) and d), with c) in a maximalconcentration of 30 wt. % and d) in a maximal concentration of 10 wt. %.

Monomers c) thereby represent one or more ethylenically unsaturatedester compounds of formula (III)

wherein R is equal to H or CH₃, R⁷ represents a linear or branched alkylgroup with 16 to 30 carbon atoms and R⁸ and R⁹ independently represent Hor a group of the formula —COOR′″, wherein R′″ is H or an alkyl groupwith 16 to 30 carbon atoms.

Examples of component c) are, among others, (meth)acrylates, fumaratesand maleates, which derived from saturated alcohols such as2-methylhexadecyl (meth)acrylate, heptadecyl (meth)acrylate,5-isopropylheptadecyl (meth)acrylate, 4-tert-butyloctadecyl(meth)acrylate, 5-ethyloctadecyl (meth)acrylate, 3-isopropyloctadecyl(meth)acrylate, octadecyl (meth)acrylate, nonadecyl (meth)acrylate,eicosyl (meth)acrylate, cetyleicosyl (meth)acrylate, stearyleicosyl(meth)acrylate and/or docosyl (meth)acrylate.

Optionally, the polyalkyl(meth)acrylate viscosity index improver (i)contains 5 to 20 wt. % of the monomers a), 70 to 90 wt. % of themonomers b) and 2 to 25 wt. % of the monomers c) in polymerized form.

Monomers d) are at least one N-dispersant monomer. Preferred thisN-dispersant monomer is of the formula (IV)

wherein R¹⁰, R¹¹ and R¹² independently are H or an alkyl group with 1 to5 carbon atoms and R¹³ is either a group C(Y)X—R¹⁴ with X═O or NH and Yis (═O) or (═NR¹⁵), where R¹⁵ is an alkyl or aryl group. R¹⁴ representsa linear or branched alkyl group with 1 to 20 carbon atoms which issubstituted by a group NR¹⁶R 17 , where R¹⁶ and R¹⁷ independentlyrepresent H or a linear or branched alkyl group with 1 to 8 carbonatoms, or wherein R¹⁶ and R¹⁷ are part of a 4 to 8 membered saturated orunsaturated ring containing optionally one or more hetero atoms chosenfrom the group consisting of nitrogen, oxygen or sulfur, wherein saidring may be further substituted with alkyl or aryl groups.

Alternatively R¹³ is a group NR¹⁸R¹⁹, wherein R¹⁸ and R¹⁹ are part of a4 to 8 membered saturated or unsaturated ring, containing at least onecarbon atom as part of the ring which forms a double bond to a heteroatom chosen from the group consisting of nitrogen, oxygen or sulfur,wherein said ring may be further substituted with alkyl or aryl groups.

Preferably, said dispersant monomer d) of polymer (i) is at least onemonomer selected from the group consisting of N-vinylic monomers,(meth)acrylic esters, (meth)acrylic amides, (meth)acrylic imides eachwith N-containing, dispersing moieties in the side chain. In particularit is preferred that the N-dispersant monomer is at least one monomerselected from the group consisting of N-vinyl pyrrolidone,N,N-dimethylaminoethyl methacrylate andN,N-dimethylaminopropylmethacrylamide.

Optionally the polyalkyl(meth)acrylate viscosity index improver (i)contains 5 to 25 wt. % of the monomers c) and 1 to 7 wt. % of themonomers d), both in polymerized form. Especially the viscosity indeximprover (i) contains 10 to 20 wt. % of the monomers c) and 2 to 5 wt. %of at least one N-dispersant monomer d) in polymerized form.

For this invention the base oil (ii) is selected from API group I, II,III or IV base oils or a mixture thereof. By using one of these baseoils or mixtures of at least two of these base oils together with theviscosity index improver (VII) described above the formulated hydraulicfluid of this invention has a fresh oil viscosity index of at least 160,a viscosity at 40° C. of 15 cSt to 51 cSt and a density at 15° C. of 800kg/m³ to 890 kg/m³. Especially preferred are API group IV base oils inform of polyalphaolefin (PAO) or mixtures of API group I to IV base oilscontaining at least 50 wt. % polyalphaolefins.

Synthetic hydrocarbons, especially polyolefins are well known in the artas API group IV base oils. These compounds are obtainable bypolymerization of alkenes, especially alkenes having 3 to 12 carbonatoms, like propene, 1-hexene, 1-octene, 1-decene and 1-dodecene, ormixtures of these alkenes. Preferred PAOs have a number averagemolecular weight in the range of 200 to 10000 g/mol, more preferably 500to 5000 g/mol.

In particular the hydraulic fluid composition comprises 70 to 95 wt. %,more preferably 80 to 95 wt. % and even more preferably 80 to 90 wt. %of the base oil (ii) selected from API group I, II, III or IV base oilsor mixture thereof and 5 to 30 wt. %, more preferably 5 to 20 wt. % andeven more preferred 10 to 20 wt. % of the polyalkyl(meth)acrylateviscosity index improver (i). Especially suitable are hydraulic fluidscorresponding to this invention having a viscosity index of at least180, preferred of at least 200, especially preferred of at least 250 anda viscosity at 40° C. of 15 cSt to 36 cSt, preferred between 15 cSt and28 cSt, especially preferred between 19 cST and 28 cST. Furthermore itis advantageous, if the hydraulic fluid has a density at 15° C. of 800kg/m³ to 860 kg/m³, preferred of 800 kg/m³ to 840 kg/m³.

By calculating the hydraulic fluid composition it has to be consideredthat the viscosity index improver (VII) might be added in a solvent. Ina preferred embodiment of this invention this solvent is also an APIgroup I, II, III or IV oil. It is especially preferred that this solventis identical to the base oil of the composition. Independently from thesolvent that is used here it has to be calculated as part of the baseoil in the composition. Usually the VII solution that is added contains20 to 40 wt. % solvent.

The viscosity index can be determined according to ASTM D 2270.

The hydraulic fluid composition according to this invention may alsocontain a Dispersant-Inhibitor package (DI package) to improveparameters like foam, corrosion, oxidation, wear and others. This DIpackage may comprise antioxidants, antifoam agents, anticorrosion agentsand/or at least one Phosphorous or Sulfur containing antiwear agent.

Technical Benefits of This Invention

High VI hydraulic fluids are typically applied in mobile applicationssuch as excavators. In these applications the hydraulic fluid has todeal with a broad variety of temperatures—very low starting temperaturesin winter and very high temperatures under heavy load conditions. Thehigh VI of the fluid is required to keep the viscosity as close aspossible to the optimum. The optimum is defined by the balance betweenmechanical efficiency which requires a thin oil and volumetricefficiency which requires a thick oil to minimize losses by internalleakage in the pump. In regular operating conditions and especiallyunder heavy load conditions volumetric efficiency becomes the dominantfactor and the viscosity index improver can greatly improve theefficiency by increasing the viscosity of the fluid.

The injection molding application is completely different compared to anexcavator. The outside temperature is constant, the work cycle is welldefined and heavy load conditions are avoided if possible. For thisreason the oil temperature is rather constant and high VI base fluidsare generally not used. Usually ISO46 monograde fluids are recommendedby the producers of injection molding machines.

For these reasons it would not be expected to see an advantage of highVI fluids in an application as injection molding, but surprisingly wefound significant energy savings when low-viscosity hydraulic fluidswith high VI were used. Completely opposed to the well-described energysavings with high VI fluids in excavators the efficiency increase ininjection molding is largest under low load conditions.

Surprisingly said method as defined above respectively in claim 1 notonly achieves the above-mentioned objectives, but also advantageouslyprovides an increased oil life time with consequent longer drainintervals for the hydraulic system.

Furthermore, the system performance of the hydraulic system can beimproved. The expression system performance means the work productivitybeing done by the hydraulic system within a defined period of time.Particularly, the system performance can be improved at least 5%, morepreferably at least 10%. In preferred systems, the work cycles per hourcan be improved.

Synthesis of the Viscosity Index Improver

For the synthesis of the polyalkyl(meth)acrylate viscosity indeximprover (i) the monomer mixtures described above can be polymerized byany known method. Conventional radical initiators can be used to performa classic radical polymerization. These initiators are well known in theart. Examples for these radical initiators are azo initiators like2,2′-azodiisobutyronitrile (AIBN), 2,2′-azobis(2-methylbutyronitrile)and 1,1 azo-biscyclohexane carbonitrile; peroxide compounds, e.g. methylethyl ketone peroxide, acetyl acetone peroxide, dilauryl peroxide,tert.-butyl per-2-ethyl hexanoate, ketone peroxide, me-thyl isobutylketone peroxide, cyclohexanone peroxide, dibenzoyl peroxide, tert.-butylper-benzoate, tert.-butyl peroxy isopropyl carbonate,2,5-bis(2-ethylhexanoyl-peroxy)-2,5-dimethyl hexane, tert.-butyl peroxy2-ethyl hexanoate, tert.-butyl peroxy-3,5,5-trimethyl hexanoate,dicumene peroxide, 1,1 bis(tert. butyl peroxy) cyclohexane, 1,1bis(tert. butyl peroxy) 3,3,5-trimethyl cyclohexane, cumenehydroperoxide and tert.-butyl hydroperoxide.

Poly(meth)acrylates with a lower molecular weight can be obtained byusing chain transfer agents. This technology is ubiquitously known andpracticed in the polymer industry and is described in Odian, Principlesof Polymerization, 1991.

Furthermore, novel polymerization techniques such as ATRP (Atom TransferRadical Polymerization) and or RAFT (Reversible Addition FragmentationChain Transfer) can be applied to obtain useful polymers derived fromalkyl esters. These methods are well known. The ATRP reaction method isdescribed, for example, by J-S. Wang, et al., J. Am. Chem. Soc., Vol.117, pp. 5614-5615 (1995), and by Matyjaszewski, Macromolecules, Vol.28, pp. 7901-7910 (1995). Moreover, the patent applications WO 96/30421,WO 97/47661, WO 97/18247, WO 98/40415 and WO 99/10387 disclosevariations of the ATRP explained above to which reference is expresslymade for purposes of the disclosure. The RAFT method is extensivelypresented in WO 98/01478, for example, to which reference is expresslymade for purposes of the disclosure.

The polymerization can be carried out at normal pressure, reducedpressure or elevated pressure. The polymerization temperature is alsonot critical. However, in general it lies in the range of −20 to 200°C., preferably 60 to 120° C., without any limitation intended by this.The polymerization can be carried out with or without solvents. The termsolvent is to be broadly understood here. According to a preferredembodiment, the polymer is obtainable by a polymerization in API GroupI, II or III mineral oil or in API group IV synthetic oil.

EXAMPLES

The invention is illustrated further in the following non-limitingexample and the comparative example (reference oil). The example belowserves for further explanation of preferred embodiments according to thepresent invention, but are not intended to restrict the invention. Allresults are shown in Table 1 and Table 2.

Testing and Oils

For determining the energy consumption, different test oils werecompared with a reference (ISO VG 46 monograde Castrol Hyspin DF Top 46,VI=100).

The following hydraulic fluids are used:

TABLE 1 Hydraulic fluid formulations density KV₄₀ KV₁₀₀ @15° C.Formulation [mm²/s] [mm²/s] VI [kg/L] Comparative  0.85% Hitec 521 Freshoil: 46.6 7.6 131 0.847 Example 1    8% Nexbase 3060 Fill for trial:46.6 7.6 130 91.15% Nexbase 3080 After trial: 46.5 7.6 130 ComparativeAral Forbex SE Fresh oil: 47.2 8.2 148 0.973 Example 2 Fill for trial:47.0 8.2 149 After trial: 46.9 8.2 149 Reference Castrol Hyspin Freshoil: 45.7 6.7 100 0.873 Oil DF Top 46 Fill for trial: 45.7 6.7 100 Aftertrial: 45.7 6.7 100 Example 1  5.8% PAMA-1 Fresh oil: 46.3 8.4 160 0.851 0.85% Hitec 521 Fill for trial: 46.3 8.4 160   21% Nexbase 3080 Aftertrial: 46.1 8.3 158 72.35% Nexbase 3060 Example 2  14.2% PAMA-1 Freshoil: 46.6 9.8 203 0.853  0.85% Hitec 521 Fill for trial: 46.2 9.7 20117.45% Nexbase 3060 After trial: 46.0 9.6 200  67.5% Nexbase 3043Example 3  8.8% PAMA-1 Fresh oil: 32.0 7.0 189 0.842 DI package Fill fortrial: 32.1 7.0 189 Nexbase 3043 + 3060 After trial: 32.3 7.0 189Example 4   20% PAMA-2 Fresh oil: 25.7 7.5 285 0.831 DI package Fill fortrial: 25.8 7.5 283 PAO-2 After trial: 25.7 7.4 281

The polyalkylmethacrylate viscosity index improver PAMA-1 consists of 13wt. % of methyl methacrylate and 87 wt. % of C₁₂₋₁₄ alkyl methacrylates(M_(w)=52,000 g/mol, PDI=2.1), dissolved in highly refined mineral oil.

The polyalkylmethacrylate viscosity index improver PAMA-2 consists of 10wt. % of methyl methacrylate and 90 wt. % of C₁₂₋₁₅ alkyl methacrylates(M_(w)=58,000 g/mol, PDI=2.0), dissolved in highly refined mineral oil.

Properties Method Kinematic viscosity at 40° C., ASTM D445 mm²/sKinematic viscosity at 100° C., ASTM D445 mm²/s VI ASTM D2270 Density at15° C., kg/L ASTM D1298

The injection molding machine that was used to create the data wasKrauss Maffei KM 80/380 CX. The energy consumption of the hydraulic pumpwas calculated by measuring voltage and current of the pump motor withexternal test equipment (measuring amplifier MX 840 PAKAP; element forvoltage recording MX 403 B, 1000V; both from Hottinger BaldwinMesstechnik GmbH). Before testing the system was flushed with thehydraulic fluid to be used and the oil parameters were checked to ensurethat the previous oil was properly purged and no mixing with previousoils occurred. Table 1 shows viscometric data for fresh oils, oil fillfor trial and for the oil collected after the trial.

During testing, molding cycles were run with a PLEXIGLAS®-moldingcompound which was, in cycle A, covered with CoverForm® Reactive-Liquidcf30OA monomer mixture.

The evaluation of data has focused on process steps without polymer toavoid any influence of polymer properties on the results.

FIG. 1: Description of a typical injection molding cycle

The cycle begins when the mold closes (Step 1), followed by building upa pressure (Step 2 a) which is required to keep the mold closed duringinjection. After moving the extruder to the mold (Step 2 b), material isinjected (Step 3) and a working pressure is maintained to compensatematerial shrinkage during molding (Step 4). Optionally, the work piececan be coated with a CoverForm® process step (Step 4.1, applied in CycleA). The extruder is moved back when the cooling phase has started (Steps5 and 6). At the end of the cooling phase the mold is opened (Step 7)and the work piece can be removed (Step 8).

Table 2 shows the differences in energy consumption (savings arenegative values) found for cycle A, cycle B and an evaluation of Step 1and Step 2 taken from cycle A data.

Step 1+Step 2 (2a+2b)+Step 4.1+Step 7+Step 8   Cycle A

Step 1+Step 2 (2a+2b)+Step 7+Step 8   Cycle B

Within this cycle, Steps 1, 2, 4.1, 7 and 8 are independent of thematerial which is injected. Consequently, the energy savings areindependent on the plastic material properties.

The coating step 4.1 is optional and part of the CoverForm process.Cycle A (with coating) and cycle B (without coating) evaluate theinfluence of this step on energy savings.

TABLE 2 Differences in energy consumption with investigated hydraulicfluids Comparative Comparative Ex 1 Ex 2 Ex 1 Ex 2 Ex 3 Ex 4 Δ energyconsumption versus reference oil [%] Cycle A — 3.6 −4.9 −7.5 −6.7 —Cycle B 2.5 5.1 −5.4 −7.9 −5.2 −9.5 Step 1 + Step 2 — 2.1 −7.0 −8.6 −5.7— Cycle A: process steps which are material independent, withCoverForm ® process step Cycle B: process steps which are materialindependent, without CoverForm ® process step Step 1 +Step 2: fullymaterial independent steps before material injection

On the basis of the above results, it is clearly demonstrated thatphysical parameters of the base oil in combination with a viscosityindex improver as defined in claim 1 are crucial in order to observeenergy savings in an hydraulic system used under the high pressureconditions of a plastic injection molding process.

Although illustrated and described herein with reference to certainspecific embodiments, the present invention is nevertheless not intendedto be limited to the details shown. Rather, various modifications may bemade in the details within the scope of the claims.

1. A hydraulic fluid composition used for reducing the energyconsumption of a hydraulic system, comprising a hydraulic fluid, in aplastic injection molding process, wherein the hydraulic fluidcomposition comprises (i) a polyalkyl(meth)acrylate viscosity indeximprover comprising monomer units of a) 5 to 40 wt. % of one or moreethylenically unsaturated ester compounds of formula (I)

wherein R is equal to H or CH₃, R¹ represents a linear or branched alkylgroup with 1 to 6 carbon atoms, R² and R³ independently represent H or agroup of the formula —COOR′, wherein R′ is H or an alkyl group with 1 to5 carbon atoms, b) 50 to 95 wt. % of one or more ethylenicallyunsaturated ester compounds of formula (II)

wherein R is equal to H or CH₃, R⁴ represents a linear or branched alkylgroup with 7 to 15 carbon atoms, R⁵ and R⁶ independently represent H ora group of the formula —COOR″, wherein R″ is H or an alkyl group with 6to 15 carbon atoms, c) 0 to 30 wt. % of one or more ethylenicallyunsaturated ester compounds of formula (III)

wherein R is equal to H or CH₃, R⁷ represents a linear or branched alkylgroup with 16 to 30 carbon atoms, R⁸and R⁹ independently represent H ora group of the formula —COOR′″, wherein R′″ is H or an alkyl group with16 to 30 carbon atoms, and d) 0 to 10 wt. % of at least one N-dispersantmonomer, and (ii) a base oil selected from API group I, II, III or IVbase oils or mixture thereof, wherein the formulated hydraulic fluid hasa fresh oil viscosity index of at least 160, a viscosity at 40° C. of 15cSt to 51 cSt, a density at 15° C. of 800 kg/m³ to 890 kg/m³.
 2. Thehydraulic fluid composition according to claim 1, wherein the industrialhydraulic application is a plastic injection molding process or is aprocess carried out in a hydraulic press.
 3. The hydraulic fluidcomposition according to claim 1, wherein the weight average molecularweight (M_(w)) of the polyalkyl(meth)acrylate viscosity index improver(i) is 20,000 to 100,000 g/mol.
 4. The hydraulic fluid compositionaccording to claim 3, wherein the weight average molecular weight(M_(w)) of the polyalkyl(meth)acrylate viscosity index improver (i) is40,000 to 70,000 g/mol.
 5. The hydraulic fluid composition according toclaim 1, wherein the hydraulic fluid has a viscosity index of at least180, a viscosity at 40° C. of equal or less than 36 cSt and a density at15° C. of less than 860 kg/m³.
 6. The hydraulic fluid compositionaccording to claim 5, wherein the hydraulic fluid has a viscosity indexof at least 250, a viscosity at 40° C. between 19 cSt and 28 cSt and adensity at 15° C. of less than 840 kg/m³.
 7. The hydraulic fluidcomposition according to claim 1, wherein said N-dispersant monomer isof the formula

wherein R¹⁰, R¹¹ and R¹² independently are H or an alkyl group with 1 to5 carbon atoms and R¹³ is either a group C(Y)X—R¹⁴ with X═O or X═NH andY is (═O) or (═NR¹⁵), where R¹⁵ is an alkyl or aryl group, and R¹⁴represents a linear or branched alkyl group with 1 to 20 carbon atomswhich is substituted by a group NR¹⁶R¹⁷ where R¹⁶ and R¹⁷ independentlyrepresent H or a linear or branched alkyl group with 1 to 8 carbonatoms, or wherein R¹⁶ and R¹⁷ are part of a 4 to 8 membered saturated orunsaturated ring containing optionally one or more hetero atoms chosenfrom the group consisting of nitrogen, oxygen or sulfur, wherein saidring may be further substituted with alkyl or aryl groups, or R¹³ is agroup NR¹⁸R¹⁹, wherein R¹⁸ and R¹⁹ are part of a 4 to 8 memberedsaturated or unsaturated ring, containing at least one carbon atom aspart of the ring which forms a double bond to a hetero atom chosen fromthe group consisting of nitrogen, oxygen or sulfur, wherein said ringmay be further substituted with alkyl or aryl groups.
 8. The hydraulicfluid composition according to claim 7, wherein said dispersant monomere) of polymer (i) is at least one monomer selected from the groupconsisting of N-vinylic monomers, (meth)acrylic esters, (meth)acrylicamides, (meth)acrylic imides each with dispersing moieties in the sidechain.
 9. The hydraulic fluid composition according to claim 7, whereinsaid N-dispersant monomer is at least one monomer selected from thegroup consisting of N-vinyl pyrrolidone, N,N-dimethylaminoethylmethacrylate, N,N-dimethylaminopropylmethacrylamide.
 10. The hydraulicfluid composition according to claim 1, wherein thepolyalkyl(meth)acrylate viscosity index improver comprises apolydispersity index of between 1.5 and 2.5.
 11. The hydraulic fluidcomposition according to claim 1, wherein the polyalkyl(meth)acrylateviscosity index improver comprises monomer units of a) 5 to 20 wt. % ofthe compounds of formula (I), b) 70 to 90 wt. % of the compound offormula (II), and c) 5 to 25 wt. % of the compound of formula (III). 12.The hydraulic fluid composition according to claim 1, wherein thehydraulic fluid composition comprises 70 to 95 wt. % of the base oilselected from API group I, II, III or IV base oils or mixture thereofand 5 to 30 wt. % of the polyalkyl(meth)acrylate viscosity indeximprover.
 13. The hydraulic fluid composition according to claim 12,wherein the hydraulic fluid composition comprises 80 to 95 wt. % of thebase oil and 5 to 20 wt. % of the polyalkyl(meth)acrylate viscosityindex improver.
 14. The hydraulic fluid composition according to claim1, wherein the hydraulic fluid composition comprises aDispersant-Inhibitor (DI) package, comprising antioxidants, antifoamagents, anticorrosion agents and/or at least one Phosphorous or Sulfurcontaining antiwear agent.
 15. The hydraulic fluid composition accordingto claim 2, wherein the weight average molecular weight (M_(w)) of thepolyalkyl(meth)acrylate viscosity index improver (i) is 20,000 to100,000 g/mol.
 16. The hydraulic fluid composition according to claim 2,wherein the hydraulic fluid has a viscosity index of at least 180, aviscosity at 40° C. of equal or less than 36 cSt and a density at 15° C.of less than 860 kg/m³.
 17. The hydraulic fluid composition according toclaim 2, wherein said N-dispersant monomer is of the formula

wherein R¹⁰, R¹¹ and R¹² independently are H or an alkyl group with 1 to5 carbon atoms and R¹³ is either a group C(Y)X—R¹⁴ with X═O or X═NH andY is (═O) or (═NR¹⁵), where R¹⁵ is an alkyl or aryl group, and R¹⁴represents a linear or branched alkyl group with 1 to 20 carbon atomswhich is substituted by a group NR¹⁶R¹⁷ where R¹⁶ and R¹⁷ independentlyrepresent H or a linear or branched alkyl group with 1 to 8 carbonatoms, or wherein R¹⁶ and R¹⁷ are part of a 4 to 8 membered saturated orunsaturated ring containing optionally one or more hetero atoms chosenfrom the group consisting of nitrogen, oxygen or sulfur, wherein saidring may be further substituted with alkyl or aryl groups, or R¹³ is agroup NR¹⁸R¹⁹, wherein R¹⁸ and R¹⁹ are part of a 4 to 8 memberedsaturated or unsaturated ring, containing at least one carbon atom aspart of the ring which forms a double bond to a hetero atom chosen fromthe group consisting of nitrogen, oxygen or sulfur, wherein said ringmay be further substituted with alkyl or aryl groups.
 18. The hydraulicfluid composition according to claim 8, wherein said N-dispersantmonomer is at least one monomer selected from the group consisting ofN-vinyl pyrrolidone, N,N-dimethylaminoethyl methacrylate,N,N-dimethylaminopropylmethacrylamide.
 19. The hydraulic fluidcomposition according to claim 2, wherein the polyalkyl(meth)acrylateviscosity index improver comprises a polydispersity index of between 1.5and 2.5.
 20. The hydraulic fluid composition according to claim 2,wherein the polyalkyl(meth)acrylate viscosity index improver comprisesmonomer units of a) 5 to 20 wt. % of the compounds of formula (I), b) 70to 90 wt. % of the compound of formula (II), and c) 5 to 25 wt. % of thecompound of formula (III).